THE ANGIOSPERMAH : LEAVES 



961 



in entire leaves, being determined by the growth pattern of the leaf as 

 a whole. 



Heterophylly. 



It is no uncommon thing to hnd leaves of different types on one and the 

 same plant, a phenomenon known by the general name of heterophylly, 

 although the features of the condition are by no means uniform. In the 

 widest sense of the term, and including as leaves all the various categories of 

 bracts, cataphylls, etc., one might say that heterophylly is the universal rule, 

 but confining ourselves only to modifications of the foliage leaves, we may 



Fig. 949. — Ranunculus peltatus. An example of environmental 

 heterophylly. The submerged leaves are filamentous. 



distinguish at least three main types of heterophylly. The first type we may 

 call environmental heterophylly, because the difi'erence is conditioned 

 by environmental factors. The clearest examples of this are to be seen among 

 w^ater plants, where the submerged and the floating or aerial leaves may be 

 very distinct. Among dicotyledonous aquatics the submerged leaves are 

 often so much subdivided that their segments are reduced to mere filaments, 

 while the aerial leaves have a fully developed lamina (Fig. 949). Monoco- 

 tyledonous aquatics on the other hand frequently have linear submerged 

 leaves with no lamina. Certain plants, especially shrubs growing in dry 

 climates, may also show a variation of leaf form according to the season, 

 the normal leaves only appearing during the rainy season and being substituted 

 by scale-leaves during the dry part of the year. We shall refer to these 

 environmental relationships in Volume IV. Closely associated with the 

 above instances are those in which certain leaves are specially modified 

 from the normal tvpe in connection with some special function, such as 



